Solid-state diffusion process for the manufacture of permanent magnet alloys of transition elements and metals of the rare-earth group

ABSTRACT

For the manufacture of permanent magnet alloys containing transition elements and rare-earth metals, the alloying operation usually gives rise to considerable difficulties. An improved alloying process is provided, which includes induction melting of the alloy constituents in a refractory crucible, and wherein the transition elements are melted by successive stages, the resulting alloy being cooled at the commencement of its solidification and the rare-earth metals being added through direct diffusion in the cooled alloy. This process is of special interest in the manufacture of a cobalt-samarium alloy. The attack of the crucible by the samarium is avoided, and the loss of samarium through evaporation is reduced.

United States Patent [191 Haberer et a1.

SOLID-STATE DIFFUSION PROCESS FOR THE MANUFACTURE OF PERMANENT MAGNET ALLOYS OF TRANSITION ELEMENTS AND METALS OF THE RARE-EARTH GROUP Inventors: Jean-Paul Haberer, Saint Martin dHeres; Roger Noel Fontaine, Montbonnot. both of France Societe DEtudes Et De Recherches Magnetiques (S.E.R.M.A.G.), Saint Martin DHeres, France Filed: Dec. 10, 1971 Appl. N0.: 206,732

Assignee:

Foreign Application Priority Data Dec. 10. 1970 France 70.44447 U.S. Cl.... 148/101, 148/31.57, 148/100, 148/103, 75/170 Int. Cl. H011 l/02 Field of Search 148/100, 103, 105, 31.57; 164/251, 56, 57, 58, 66;75/135, 170, 142, 152, 171, 122

References Cited UNITED STATES PATENTS 7/1953 Ebeling. 75/171 l/l963 Knapp et al. 75/152 7/l965 I 75/170 H1969 Ostertag et al. 75/170 June 11, 1974 3,544,312 12/1970 Turillon et al, 75/170 3,560,200 2/1971 Nesbitt et a1. 75/122 3.625.779 12/1971 Cech 148/101 3.655.463 4/1972 Benz 148/101 FOREIGN PATENTS OR APPLICATIONS 751,294 l/l967 Canada 164/58 7 OTHER PUBLICATIONS Nassau, K. et al., lntermetallic Compounds Between Lanthanons and Transition Metals. J. Phys Chem. Soc.. 1960. (16) Pp- 123-130.

Primary E.raminerL. Dewayne Rutledge Assistant ExaminerW. R. Satterfield [57] ABSTRACT This process is of special interest in the manufacture of a cobalt-Samarium alloy. The attack of the crucible by the samarium is avoided, and the loss of Samarium through evaporation is reduced.

3 Claims, 2 Drawing Figures PATENTEDJUNH mm 3816ll89 sum 1 or 2 SOLID-STATE DIFFUSION PROCESS FOR THE MANUFACTURE OF PERMANENT MAGNET ALLOYS OF TRANSITION ELEMENTS AND METALS OF THE RARE-EARTH GROUP The present invention relates to the manufacture of permanent magnet materials based on alloys of transition elements (generally cobalt, possibly with additions of iron and/or copper and/or nickel) and metals of the group comprising the rare-earth metals and yttrium and scandium.

These materials and their processes'of manufacture have more particularly been described in an article of Karl J. STRNAT, entitled Cobalt-rare-earth metal alloys, materials of the future for permanent magnets," which was published in the periodical Cobalt of September 1967 (pages 133-143).

Their magnetic properties are of exceptional interest but their manufacture on an industrial scale gives rise to considerable practical difficulties.

This manufacture comprises the preparation, by an alloying operation followed by grinding, of a powder having the required composition, and the consolidation of this powder for forming a magnet, consolidation being carried out, for example, by compression followed by sintering.

The present invention concerns the alloying operation. Fusion of the alloy components is mostly carried out in an arc furnace of the button type, in a protective atmosphere of inert gas or in a vacuum, or again by induction in a'refractory crucible of pure alumina,

for example, or in a suspension melting furnace.

The button furnace has more particularly the following disadvantages: the charge of each button is too small; homogeneity of the alloy is too uncertain; risk of projections involving loss of constituent elements of the alloy; risk of local overheating resulting in nonreproducible evaporation; no possibility of quenching.

Suspension or levitation furnaces have too small a charge and the losses in them are uncontrollable.

The invention proposes to remedy these disadvantages. It relates more particularly to the process of induction melting in a refractory crucible. This process is particularly simple to carry out, but in the prior art it has the disadvantage of necessitating the use of a high quality, and therefore expensive, crucible. Indeed, the rare-earth metals forming part of the composition of the alloy and in particular the samarium, are generally strongly reducing with respect to the crucible material, which they destroy relatively rapidly.

The present invention proposes to eliminate this disadvantage by carrying out melting of the transition element or elements in a first stage. then after allowing the said element or elements to cool to the commencement of solidification, in, a second stage, of the addition of the rare earth metal or metals by direct diffusion in the transition element or alloy of transition elements in the solid state.

According to one particular embodiment, in the case where the alloy is intended to be magnetically hardened in the solid state by precipitation of a weakly magnetic second phase (rich in copper for example) in the grains of a strongly magnetic matrix (rich in cobalt), the solidification of its constituents after casting takes place in an enclosure comprising relatively very hot walls and a relatively cold bottom.

of the following description.

In the accompanying drawings:

F IG. 1 represents a simple device for carrying out the process according to the invention, used by way of example in the case of a cobalt-samarium alloy containing a certain proportion of copper and iron, and

FIG. 2 represents in longitudinal section a modification used, for example, in the preparation of a material not comprising a weakly magnetic second phase.

FIG. 1 shows, arranged in the interior of an enclosure 1 containing argon at a pressure of 400 torr., a crucible 2 of recrystalized alumina and an ingot mould 3.

The enclosure is provided with an opening 4 through which is introduced a samarium rod 5 suspended from a wire 6 provided with a handle 7.

The crucible 2 is surrounded by a heating coil 8, through which it is possible to send a high-frequency current.

The ingot mould 3 is surrounded by a heating coil 3a. Furthermore, its base 3b is cooled by any means, not shown. For example, it is sufficient if it is made of a plate of a metal which is a good conductor of heat, such as iron.

As a modification, the sides of the ingot mould may be made of a sand containing an inflammable mixture. They could be ignited before casting, or again they could be left to ignite spontaneously during casting of the alloy (exothermic" mould, known per se).

There is produced in a first stage the melting of the cobalt, copper and iron, previously placed in the crucible, by heating to about 1500C. It is then allowed to cool to 1 C and the samarium rod is then lowered until it comes into contact with the contents of the crucible, the temperature being kept at 1 100C. Slow diffusion of the samarium occurs in the cobalt in the solid state. This process not only obviates attack of the crucible by the samarium, but furthermore considerably limits the evaporation of this latter constituent.

When the alloyingoperation is terminated, the alloy I is reheated slightly to liquefy it, then the crucible is tilted, as indicatedin dashed lines, pouring its contents into the ingot mould. The alloy solidifies with columnar crystallization due to presence of the hot walls and a cold bottom forming a thermal gradient mould.

FIG. 2 shows an apparatus serving for example, for the preparation of an alloy on a samarium and cobalt base.

This apparatus comprises an alumina crucible 9 surrounded by a high-frequency heating coil 9a, an ingot mould l0 and a samarium magazine ll.

Arranged between the crucible 9 and coil 9a is a heat-insulating enclosure 12 having in its upper part a circular half-cover provided with an orifice (not shown) and, in its lower part a bottom provided with an 3 orifice 12a situated opposite an orifice 9b of the crucible.

The orifice 9b is normally closed by an alumina rod 13 movable in translation and rotation. The magazine 11 is secured to this rod at 11a and 11b. It is formed of a quartz tube, the lower end of which rests on the halfcover 120. In the normal position of the rod, the lower end of the tube 11 is not opposite the orifice, with which the half-cover is provided, so that the samarium remains in the magazine. The rod may be rotated until the tube 11 communicates with the crucible by means of the said orifice; the samarium then descends into the crucible by gravity.

The enclosure 12 rests on a brick support 14 pro- .vided at its centre with an orifice opposite the receptacle of the ingot mould. The latter is cooled by circulation of water in a pipe line a, I

The cobalt is introduced first into the crucible and its melting is carried out by passing current through the coil. The alloy thus obtained is cooled to l-lOOC, and the rod 13 is manipulated by rotation to produce the introduction of the samarium into the crucible. The slow diffusion of the samarium results in the formation of the complete alloy which is to be obtained. The rod 13 is then moved in translation to release the orifice 9b and permit the alloy to flow into the ingot mould.

As a modification, the apparatus of FIG. 2 could be adapted for columnar crystallization. For this purpose, it would merely be necessary for the ingot mould to be cooled only at the bottom, and for the coil 9a to be lowered at the moment of pouring so as to surround the side wall of the ingot mould and heat the said wall.

Likewise, the apparatus of FIG. 1 could be used for preparing an alloy not comprising a second, weakly magnetic phase. The ingot mould would then have to be cooled througout its entire mass, and would be for example similar to that ofthe apparatus of FIG. 2.

Furthermore, the constructional details of the apparatuses and processes as described could be varied, without departing from the spirit of the invention.

We claim:

1. An improved process for manufacturing a permanent magnet-type alloy composed of cobalt or an alloy thereof and samarium in a refractory crucible without substantial reaction with and degradation of said crucible, said process characterized by forming said alloy in the solid state and below the melting point thereof and comprising the steps of:

a. introducing cobalt or an alloy thereof into said crucible;

b. heating said crucible byinduction until the temperature of the contents thereof is about 1500 C. and a liquid melt is formed;

0. discontinuing said heating and allowing said crucible to cool to a temperature of the order of about 1100 C at which temperature said contents are solid;

d. contacting the surface of said solid with samarium metal in the solid state at saidtemperature and forming said permanent-magnet type alloy by diffusion of said samarium into said cobalt in the solid state, said solid samarium metal slowly diffusing into said cobalt metal, while avoiding substantial evporation of said samarium and thereby producing said permanent magnet-type alloy in the solid state.

2. The process as claimed in claim 1 including the' additional steps of:

e. heating the solid permanent magnet-type alloy of step (d) to a temperature of about 1350 C. to form a melt, and thereafter,

f. pouring said melt into a mold.

3. An improved process for manufacturing a permanent magnet-type alloy composed of cobalt and samarium in a refractory crucible without substantial reaction with and degradation of said crucible, said process characterized by forming said alloy in the solid state and below the melting point thereof and comprising the steps of:

a. introducing cobalt into said crucible;

b. heating said crucible by induction until the temperature of the contents thereof is about 1 l00 C. at which temperature said contents are solid;

. contacting the surface of said solid with samarium metal in the solid state at said temperature and forming said permanent-magneti type alloy by diffusion of said samarium into said cobalt in the solid state, said solid samarium metal slowly diffusing into said solid cobalt metal, while avoiding substantial evaporation of said samarium and thereby producing said permanent-magnet-type alloy in the solidstate. 

2. The process as claimed in claim 1 including the additional steps of: e. heating the solid permanent magnet-type alloy of step (d) to a temperature of about 1350* C. to form a melt, and thereafter, f. pouring said melt into a mold.
 3. An improved process for manufacTuring a permanent magnet-type alloy composed of cobalt and samarium in a refractory crucible without substantial reaction with and degradation of said crucible, said process characterized by forming said alloy in the solid state and below the melting point thereof and comprising the steps of: a. introducing cobalt into said crucible; b. heating said crucible by induction until the temperature of the contents thereof is about 1100* C. at which temperature said contents are solid; c. contacting the surface of said solid with samarium metal in the solid state at said temperature and forming said permanent-magneti type alloy by diffusion of said samarium into said cobalt in the solid state, said solid samarium metal slowly diffusing into said solid cobalt metal, while avoiding substantial evaporation of said samarium and thereby producing said permanent-magnet-type alloy in the solid state. 